There has been proposed a related art telephone system capable of making a telephone call between a main device and a mobile device via wireless communication. In the related art telephone system, even when a telephone call is not performed with a mobile device, wireless communication is performed between a main device and the mobile device. Specifically, a synchronization signal is periodically transmitted and received between the main device and the mobile device, to keep a link between the main device and the mobile device.
Incidentally, there has been proposed a device configured to be set in a power save mode in order to save energy consumption of office automation equipment and the like which tend to have their power sources turned on for a long time has been known. In a case in which such a power save mode is applied to a cordless telephone system, for example, in a case in which there is no intention to use the cordless telephone system such as a case in which there is no plan to receive a call for a while, it is possible for a user to set the main device in a power save mode to reduce its power consumption.
In a power save mode, various types of functions of the main device may be stopped in order to save its power consumption. However, because considerable electric power is consumed in order to periodically transmit a synchronization signal to the mobile device from the main device, the transmission of a synchronization signal may be stopped when the main device makes a transition to a power save mode.
When a link between the main device and the mobile device is cut off due to such a stoppage of a synchronization signal, the mobile device judges that the mobile device itself is out of the range of the main device, and starts searches in order to receive a synchronization signal from the main device.
This search is repeatedly conducted until a synchronization signal from the main device is received to establish a link therebetween. However, in some cases, the device may be configured to stop searches in the mobile device when a state in which the mobile device does not receive a synchronization signal from the main device continues for a predetermined time or more in order to prevent the mobile device from consuming electric power.
There has been a problem that, if the mobile device stops searches in this way, even if the main device returning from a power save mode starts transmitting a synchronization signal, the synchronization signal cannot be received at the mobile device side, to be unable to establish a link therebetween. In contrast, if the mobile device is configured not to stop searches, but always execute searches, the main device and the mobile device can securely establish a link therebetween when the main device returns. However, in such a way, electric power is rapidly consumed in the mobile device.
With respect to such problems, JP-A-11-136181 discloses a following, idea. That is, in a state in which there is no call from a wireless base station, a time from when a wireless terminal station turns on its power source of a receiving unit until the wireless terminal station turns on the power source the next time is set longer.